''Connect to an external monitor.'' If you see nothing there the GPU/VGC logic board is bad. If you see an image there the internal display chain is the problem. ''' WORKING ON CRTs is VERY DANGEROUS and NOT FOR AMATEURS''' ''only a computer/small electronics repair shop should work on an all-in-one machine like this.''
You may want to use it with an external monitor'' (should that work)'' or part it out on e-bay.
'''If I misread your question''' and the "light' is the power on light, not the display - you most likely damaged the power supply. '''SAME ADVICE APPLIES''' as above.
''If this answer is acceptable '''please remember to return and mark it accepted'''.''